Thanks for the info on your setup.
There is no dual-band router that has good performance over the same range in 5 GHz as in 2.4 GHz. What you have going for you, however, is the single-floor layout, relatively small area and central location for your router.
Both the WRT610N and DIR-825 have open-air test results in the
Wireless Charts. For 5GHz streaming you want to look at the downlink tests, which I have inserted below.
The DIR-825 has a slight edge with 20MHz channel bandwidth mode, since it operated in Location D where the 610N did not. But the 610n has the edge with 40MHz channel mode, with higher throughput. In the 5 GHz band you should be able to operate with a 40MHz channel bandwidth without risking interference with other networks.
As far as upgrading antennas, you would need to use dual-band antennas, which are difficult to find, particularly in higher-gain versions.
For your wife's notebook, if you want to connect via the Ethernet port, you need a wireless bridge (sometimes called gaming adapter). Dual-band products are
D-Link DAP-1522 and
Linksys WGA600N.
However, you can also use USB client adapters like the
Linksys WUSB600N,
D-Link DWA-160 or
NETGEAR WNDA3100.